Thin film electroluminescent (TFEL) display panels are constructed using a set of transparent front electrodes, typically made of indium tin oxide (ITO), and a transparent phosphor layer sandwiched between transparent dielectric layers situated behind the front electrodes. A rear electrode set is disposed behind the rear insulating layer and is usually constructed of aluminum which provides good electrical conductivity and has a self-healing failure feature because it acts as a localized fuse at breakdown points. Aluminum also enhances the luminance of the display by reflecting back toward the viewer most of the light that would otherwise be lost to the rear of the display. While this reflected light nearly doubles the light of the displayed image, the aluminum electrode also reflects superimposed ambient light that interferes with the display information and reduces the contrast of the display.
To minimize the reflection of ambient light, an antireflection coating is typically used on the front glass. Also, dark backgrounds behind the display are commonly provided. The TFEL laminar stack is situated within an enclosure sealed against the substrate, and the rear wall of this enclosure is usually blackened to block light from extraneous light sources behind the display, and to absorb ambient light passing through the display from the front. Another method of improving the contrast and attenuating the amount of light reflected from the rear aluminum electrodes is to use an external circularly polarized contrast enhancement filter in front of the display. However, such filters can be expensive and typically attenuate the display luminance by 60% or more.
Another approach that has been tried in the past has been to use ITO transparent electrodes for the rear electrode set. This reduces reflectance and allows ambient light to pass on through to the back of the display where it can be absorbed. However, ITO is more resistive than any metallic electrodes such as those made of aluminum, and must be made much thicker to achieve adequate electrical conductivity. Thick layers of ITO do not exhibit the self-healing characteristics of aluminum rear electrodes. This leads to an unacceptable loss in device reliability due to dielectric breakdown.
In yet another approach, shown in Steel et al., U.S. Pat. No. 3,560,784, a light absorbing layer is incorporated into the thin film laminate structure. This reference suggests that if a conventional metallic rear electrode is used, then a light absorbing layer may be added as an insulating layer or as a conductive layer to achieve a black layer display. Insertion of a dark layer immediately behind the phosphor layer, however, can interfere with the phosphor/insulator interface leading to inferior display performance. The light pulse for one polarity may be reduced which can give rise to a flicker effect as well as to a loss in overall brightness.
Another approach has been to utilize a black optically absorbing layer behind the rear insulating layer and in front of the rear aluminum electrode. A similar approach is shown in a device described in U.S. Pat. No. 4,547,702 in which a dark field layer consisting of 6-10% of a noble metal, such as gold, dispersed within a ceramic, such as magnesium oxide, is used between the phosphor and rear insulator or is used as the rear insulator. In either case, the resulting luminance versus voltage characteristic is not steep enough for good matrix display operation, and a higher-than-10% gold content causes excess conductivity resulting in breakdown of the phosphor layer as well as undesirable lateral conduction between electrodes.
In yet another type of proposed device, GeNx is sandwiched as an embedded dark layer within the rear insulator. As with other structures that employ a black layer added between the phosphor layer and the rear electrode, this layer affects the dielectric properties of the insulator, and, hence the reliability of the panel with regard to dielectric breakdown.